larrysogla

With EDF...one of the goals is increased efflux(exhaust) velocity. We all know that many EDF RC fighter jets(i.e. F86, F104, F4, F16, etc.) have a thrust tube to guide the efflux out of the fuselage. The thrust tube due to friction on the inside walls is energy robbing to the efflux.......We know that in a centrifugal impeller, the design intent is to produce a higher pressure ratio than in a fan like impeller blade to better overcome the wall friction losses in the ducting(i.e. the extensive ducting in Air Conditioning have high wall friction losses so they use centrifugal impellers). So the question is.....would a curving forward swept impeller blade produce a higher pressure ratio than a standard un-swept impeller blade or trailing swept impeller blade? In my unscientific mind.....it looks like the curving forward swept impeller blade would have a higher pressure ratio. The higher pressure ratio would be more efficient in overcoming wall friction losses from the thrust tube.Also the higher pressure ratio would be more successful in accelerating the efflux velocity when some modelers squeeze the thrust tube diameter to a smaller diameter to accelerate the efflux velocity. Anyone tried this yet? What was the outcome???

BMatthews

There's much more to it than just the blade shape. For example with a tractor setup the motor housing is in the way and will leave a slower moving center to the airflow. If you set it up as a pusher style with the motor pod in front of the impeller there's still going to be a less efficient center core of the flow coming off the fan. A spinner can be used to house the inner portion but that still leaves a slower "core" to the airflow with the fastest moving air out at the outer portion. The duct can be tapered to close this down but then you're back to the wall friction again.

What you'd need is to do a whole study on a wide array of the factors to optimize the setup. The actual shape of the blades would be just one item.

pimmnz

Be careful what you you wish for.. Do you wish to increase the pressure in the duct, or the acceleration of the air? If you increase the pressure in the duct (behind the fan) then you will reduce mass flow (Thrust). To increase the flow, you must reduce the air pressure through the duct, in order to increase and encourage the air to flow through the fan and be accelerated. Air always flows from areas of higher pressure to those of lower pressure. If you 'squeeze' the exhaust duct, you will decrease the mass flow, and thrust. The most efficient fan will have the shortest duct, all else being equal.
Evan, WB #12.

larrysogla

My main concern is raising the pressure ratio with the same electric power consumed.......if that can be possible...........then we would have faster or more powerful jets for the same electric power consumption. I wonder if any EDF manufacturer have done this already(i.e. making the impeller blades a forward swept shape). If so.....was the advantage big enough to notice in improved flight performance.

pimmnz

Raising the pressure will reduce thrust...the system is a shrouded prop, not an air pump. That said, experimentation with blade shape, and in particular reduced blade tip clearance, would be a fertile area for experimentation. Maybe even a fixed shroud at the prop tips, rotating in an annular slot to maintain duct profile would be one way around the tip loss problem?
Evan, WB #12.

DanSavage

My Feedback: (5)

Already been tried. Didn't work any better than straight blades.

Bundubasher

Hi,

BFoote

Of course forward swept blades will help in regards to tip loses. Weather or not you can detect any difference on something so small is anther question entirely, let along the implementation of it.

Pressure ratio after all is essentially saying L/D ratio.

Such a design decreases tip loses after all which is a gigantic problem with small impellers and their efficiency.

Can one make the blades stiff enough that the tip loss savings aren't lost by boundary layer stripping due to vibration? The big boys can't do so. Then again with a smaller blade a higher stiffness is possible.

Next question are you thinking "standard" impeller here? Platter impeller? Contra Rotating fins? For a small impeller all of the previous are possible. No reason one could not use a platter impeller in a small EDF. A platter impeller off hand is more efficient than a small prop impeller.

Yea, its been "tried before", but the math says it should work. Now finding "unobtanium" stiffness to make it work is probably the problem in my non expert opinion.

larrysogla

One question that keeps popping up in my mind is......cavitation. Isn't the pylon racer propeller cavitating while the pylon plane is accelerating down the runway at take-off??? I will guess that the EDF impeller spinning at 40,000+ RPM is also cavitating at take-off or in a slow, steep climb. I have not seen any percentage of efficiency concerning EDF impellers. The propellers are somewhere in the 80% to 85% efficiency. What about EDF impellers......how efficient are they???? Percentage wise???

BMatthews

Airplane props do not cavitate. That's something that only happens in water where the low pressure on the one side is so high that it draws air down from the surface or actually causes the water to boil out to a gas despite the depth and pressure in the case of a powerful submarine. Airplane prop blades operate either in a stalled or unstalled condition depending on the angle of attack of the blades.

But yeah, the very high pitch of racing props means that during takeoff and until they get up close to their racing speed that the blades are operating in a stalled condition. Note that "stalled" does not mean "no lift". It's just that the lift from the prop blades that pull the model ahead is combined with a high degree of drag. This is why the engine doesn't seem to "open up" until in the air and up close to the final speed.

This same situation was also a limiting factor with the early high performance monoplane fighters and bombers and the between war racing planes. Those props had to be picked to provide a compromise between a fine enough pitch to let the plane take off within a reasonable runway length yet still be a high enough pitch to achieve a good top speed once in the air. This whole issue was the real reason for the variable pitch props being introduced.